1. Field of the Invention
This invention relates to a process for making a two-layer film carrier for TAB.
2. Description of the Prior Art
Although wire bonding has hitherto been used for interconnecting the component parts of an IC package, tape automated bonding (TAB) is now considered to be a better method when interconnections for making a multi-pinned type IC. There are three types of film carriers known for TAB. (1) Firstly, there is a one-layer film carrier formed from a single metal, such as copper or aluminum. This type of film carrier can be made at a low cost, but as a base material having a given thickness is worked on directly, it is difficult to make a finely and accurately worked product. This type of film carrier does not lend itself for use in interconnecting the parts of a multi-pinned IC, but has only a limited scope of application. (2) Secondly, there is a three-layer film carrier composed of a copper foil, a polyimide film and an adhesive by which the copper foil is bonded to the polyimide film. The use of the adhesive, however, spoils the benefits of use of the polyimide having high heat resistance and insulating property. Moreover, the manufacture of this type of film carrier has an economical disadvantage, as it requires an expensive punching tool for making sprocket and device holes. (3) Thirdly, there is a two-layer film carrier comprising a substrate which consists of a polyimide film and a metal layer formed on a surface of the polyimide film. This type of film carrier has been proposed as an improvement over the two types of film carriers as hereinabove described, and is claimed to enable the interconnections for a multi-pinned IC.
Although no details are known regarding any specific process that is used for making a two-layer film carrier, it is assumed that a process including the following steps is generally employed:
(a) Forming a photoresist coating having an appropriate thickness on a copper surface;
(b) Exposing the photoresist to light through a mask defining a desired pattern;
(c) Developing the exposed photoresist to expose the copper surface in the desired pattern;
(d) Forming a copper layer by additive plating;
(e) Dissolving and removing the remaining photoresist and removing the copper layer by flush etching;
(f) Applying a photoresist onto both sides;
(g) Exposing the photoresist on a polyimide film to light in a desired pattern;
(h) Developing the exposed photoresist to expose the selected portions of the polyimide film;
(i) Etching the exposed portions of the polyimide film;
(j) Removing the remaining photoresist from the polyimide film; and
(k) Forming a plated layer of gold or tin, if required.
This process is, however, considered to have a number of problems as will hereinafter be pointed out:
(1) As the polyimide film remains exposed to the plating solution, etc., for a long time throughout the steps (d) and (e), its property of being wetted with the photoresist becomes so low that the photoresist applied to it during step (f) is likely to peel off at any time thereafter;
(2) As the step (g) of exposure to light comes after the formation of leads, the unevenness of the surface on which the leads have been formed causes the scattering of light, resulting in a lower degree of resolution; and
(3) As the photoresists on the copper and the polyimide film are separately exposed to light, the accurate positioning of the patterns on both sides is difficult to achieve, and the operation is inefficient and is difficult to perform continuously.
These problems allow the process to produce only a two-layer film carrier which is expensive, and yet which cannot satisfactorily be used for interconnecting the parts of an IC package having a multi-pinned layer.